1. Field of the Invention
The present invention relates to a wireless communication system. More particularly, the present invention relates to a method and apparatus for transmitting a downlink frame by using two queues according to a successful transmission probability to a destination in a wireless communication system according to Institute of Electrical and Electronics Engineers (IEEE) 802.11, wireless local area network (WLAN), IEEE 802.16d/e, wireless broadband Internet (WiBro), and World Interoperability for Microwave Access (WiMAX), and the like and thereby, reducing a transmission delay of the downlink frame.
2. Description of Related Art
A fourth generation mobile communication unifies systems, such as IEEE 802.11, WLAN, IEEE 802.16d/e, WiBro, and WiMAX, and the like. In the fourth generation mobile communication, satellite networks, wireless networks, digital broadcasting networks, and video broadcasting networks are unified into a single network, and systematically interoperate with each other. Accordingly, a user may utilize a communication service such as a portable Internet service in a best state, with any network.
FIG. 1 is a diagram illustrating a conventional wireless communication system 100 environment. Referring to FIG. 1, a first host 130, a second host 140, and a third host 150 may receive a communication service, such as a call, digital broadcasting, downloading or uploading of digital medial data, and the like, via an access point (AP) 120. The first host 130, the second host 140, and the third host 150 may be a mobile phone, a notebook computer, a personal digital assistant (PDA), and the like. The AP 120 and an access router (AR) 110 are connected to each other, based on an Ethernet protocol. The AP 120 functions as a bridge for a fast connection to a host. Also, the AP 120 functions to process scheduling of wireless resources and a radio frequency (RF) control function. The AR 110 is an Internet Protocol (IP) terminating point which is mainly in charge of a layer 3 (L3), and routes IP packets so that the IP packets may be appropriately transmitted and received between the AP 120 and each of the first host 130, the second host 140, and the third host 150. The IP packets are transmitted to or received from a destination host or a destination server via the AP 120.
In the conventional wireless communication system 100 environment, the AP 120 manages a queuing unit 121, and transmits downlink frames P1, P2, P3, . . . , in a packet format, from an upper layer, such as the AR 110, to a corresponding host. For example, the frame P1 is transmitted to the first host 130, the frame P2 to the second host 140, and the frame P3 to the third host 150. In addition, the AP 120 consecutively receives frames from the upper layer and transmits the frames to a corresponding destination.
However, in the conventional wireless communication system 100 according to IEEE 802.11, WLAN, and the like, the AP 120 manages only the single queuing unit 121. Accordingly, when a transmission of a frame fails, a retransmission of the frame is attempted for a certain number of cycles. For example, as shown in FIG. 2, when a transmission of the frame P1 fails during a cycle due to a communication error with the first host 130, the AP 120 attempts a retransmission of the frame P1 during subsequent cycles. In this case, when a total number of transmission attempts becomes a certain value, for example, 7 times, but the transmission of the frame P1 fails within a time T1, the AP 120 discards the frame P1. Subsequent frames P2, P3, . . . , which are waiting to be transmitted after the frame P1, are transmitted. In this case, when it is assumed that a communication between the second host 140 and the third host 150 is good and each of the frames P2 and P3 is transmitted in only one cycle, i.e. T2 and T3, the second host 140 and the third host 150 which respectively receive the frames P2 and P3 experience a service deterioration due to a transmission delay during the time T1. In the above example, transmission delays from a transmission attempt starting time are 7 cycles for the frame P1, 8 cycles for the frame P2, and 9 cycles for the frame P3. In this case, the average of the transmission delays is (7+8+9)/3=8 cycles.
In the above example, in the case of a communication interruption with the second host 140 or the third host 150, the transmission delay may not be worse than the communication interruption with only the first host 130. However, a frame which has a comparatively low successful transmission probability in a transmission waiting line of the queuing unit 121, for example, a transmission of subsequent frames P2 and P3 may be considerably delayed.